 With this, let me show you the actual triple axis spectrometer at Dhruva. This is used for inelastic neutron flapping. I will come to this later again when I discuss specific techniques related to neutron. But here I just wanted to show you the photograph because here this large drum is the monochromotor drum. So, it contains the monochromotor at the center of it. You can see the monitor detector here which I discussed with you earlier and this is the collimator after the monochromotor. This is a goniometer you can see on the sample table. So, this goniometer because if you have a single crystal sample then you should be able to align this. After that there is a further collimator then this is the analyzer crystal. So, the analyzer crystal is sitting here and the detector rotates around the analyzer crystal and the analyzer crystal rotates around its own axis and this whole axis you can see can rotate. You can see the analyzer crystal can rotate this bottom yellow thing sorry yellow thing. It allows the rotation of the analyzer around the sample and this is a top arm. This allows the rotation of the detector around the analyzer crystal which detects the final neutron and finds out its energy and direction and this red thing is a beam catcher which is in line with the beam coming from the monochromotor drum. It's in line so that anybody travelling over here walking over here do not get any unwanted radiation. So, this is a typical triple axis spectrometer at Dhuba and such spectrometers are plenty and almost every major neutron source will have this kind of triple axis spectrometer. I told you earlier that Brockhaus, B.N. Brockhaus was the person who first introduced this concept of triple axis spectrometer and he was a Nobel Prize winner for his studies on inelastic neutron scattering. Now when I come to diffractometers which I told you here it is a much simpler geometry. I have shown you two powder diffractometers the way we in Dhuba this green table that you see and here this this blue arc actually they have position sensitive detectors inside them. There is no moving parts in these two diffractometers the beam comes on a sample which is here here it is here over here and then the diffracted beam goes into this fixed detectors over here and over here. The Q ranges this dictates actually depending on the wavelength what is the angular range and delta D by D gives us the resolution. So you can see they have got very similar Q ranges only this one has slightly better resolution because delta D by D is 0.3 percent is 0.8 percent. So this spectrometer which is powder diffractometer it is a magnetic powder diffractometer it is maintained by an organization known as UGCDE CSR and this is run by the solicit field division scientist in Abiotomic Research Center. Now this brings me to the fact that in a diffraction experiment or in any experiment one important thing is that we need to know of course the outgoing wave vector so that I know the wave vector transfer Q is equal to 4 pi by lambda sin theta and in case of in a seamless experiment along with that we also want to know the h cross omega which is the energy transfer between the neutron and the system. So we need to know the incoming energy and the outgoing energy for diffraction we do not need to know this incoming energy and outgoing energy but we need to know Q. So when I am looking for Q the fact is that sin theta I am doing a Q scanning because my aim is to get intensity as a function of Q in this experiment. So I need to scan Q that is why even the diffractometer showed that Q ranges how do I scan Q in case of a monochromatic beam lambda is fixed I keep changing theta, theta goes from it can go from few degrees to maybe 140, 150 even backscattering it can go up to 180 degree close to 180 degree in case of high resolution powder diffractometers in many spallation sources. So it is scanning by changing theta. Now we can also do the same scanning by keeping theta fixed and changing lambda. So I will be going from let us say here I go from small Q Q to large Q as I keep changing sin theta but if I go from small lambda to large lambda how I will tell you that if I do that then I go from large Q to small Q usually in case of a pulse source like a spallation neutron source the scanning is done by using a polychromatic beam so lambda is changing and the detectors are all fixed so theta is fixed. So basically I will show you later I Q versus Q if it looks like this in case of a reactor source in case of spallation source because it is the time of flight. Time of flight in the time neutron takes to travel over certain distance so when the time of flight is less neutron is more energetic so time will be less it will be a mirror image of this one I will show you the data later. So I am going from here it is large Q in this scan to small Q. So the time of flight spectrum in a spallation source is a mirror image of the intensity spectrum in a reactor source I will come back to it later. So as I told you that we can also scan Q by scanning lambda keeping the angle fixed and this is usually done in a spallation neutron source and I just show you one schematic of a spallation neutron source so here there is a 17 it is ICs at Rutherford Appleton Laboratory I am taking the example again and again because we have done lots of experiments there because today SNS at Oak Ridge is also an equally good source so anyway in this case I just give you the general concept that there is a proton beam here it is a 70 mega electron volts Linnac which feeds a 800 mega electron volt synchrotron and then the proton gets accelerated to 800 mega electron volt goes and hits the target and then there are instruments all around the target and there is one instrument which is an HRPT I will come to it later known as high resolution powder diffractometer in back scattering geometry. Now this is a schematic now spallation neutron sources has an advantage that it is not a reactor a reactor actually causes fission I mean a reactor uses fission to produce neutrons and then it is a critical assembly that means generation to generation the number of neutrons remains same and for safe reaction if it does not happen if it keeps increasing then it becomes a bomb and if it is does not if it is less than one the multiplication factor then the reactor will stop working it will be subcritical so there is subcritical critical and supercritical so we are always working in the critical region but you have to remember there are supercritical and subcritical regions and there are lots of regulations we have to follow in case of a spallation neutron source the target need not be uranium it can be a high Z material like tantalum zirconium and it need not be fissile material and of course the yield is maybe more in uranium because after spallation it can be followed by more fission by the neutrons which are produced in spallation but generally in many places for example in RL they use high Z materials like tantalum and so this is not a reactor like situation and regulations I mean much relaxed and much easier to handle but at the same time this needs a very high end technology like producing a proton beam of almost 800 mega electron volts means almost 1 giga electron volt energy so that's the challenging part and India proposes to have a spallation neutron source in future so I will get back to polarizers and time of flight and dispersive scattering a comparison in the next lecture